added @ open files context provider "pinned" option in intellij.

[uxroc]

Description

Added option to include only pinned files as context in intellij. I am attempting my first contribution as requested in continuedev/contribution-ideas#23, and willing to take the rest on the list if needed.

Checklist

• The base branch of this PR is dev, rather than main
• The relevant docs, if any, have been updated or created

Screenshots

[ "For visual changes, include screenshots." ]

Testing

[ For new or modified features, provide testing instructions. ]
added option to include only pinned files in intellij plugin as requested in
continuedev/contribution-ideas#23

As described in https://docs.continue.dev/customization/context-providers#open-files,
when onlyPinned is enabled, plugin will include only pinned files from current active project window.

[sestinj]

Thanks! Looks great